Black holes grew up before galaxies

Four galaxies in the early universe have been found that violate a previously observed relationship between the mass of a galaxy and that of the colossal black hole at its centre. The find suggests that supermassive black holes may have matured long before the galaxies that surround them instead of growing in lockstep with each other.

In recent years, evidence has built up that black holes and galaxies grow together. Black holes get bigger by gobbling up material around them. The objects also blast out tremendous radiation that can both fuel star formation and prevent it by heating up a galaxy's gas and blowing it into space before it can cool and condense into new stars.

Nearby galaxies all seem to follow an elegant but unexplained relationship: all seem to have central bulges of stars - shaped like the yolk in a fried egg - about 700 times as massive as the gargantuan black holes at their hearts. The relationship holds for a wide range of galaxy sizes and ages.

But astronomers weren't sure if this relationship would hold in the early universe. "This constant ratio indicates that the black hole and the bulge affect each others' growth in some sort of interactive relationship," said team member Dominik Riechers of Caltech in a statement. "The big question has been whether one grows before the other or if they grow together, maintaining their mass ratio throughout the entire process."

Lost in the glare

To study the early universe, astronomers must look at very distant galaxies, since the light they emitted billions of years ago is only now reaching Earth.

The most easily seen of these distant galaxies are quasars, thought to be sites where gas heats up and glows brilliantly before it plunges into a black hole as massive as a billion Suns.

Unfortunately, quasars are so bright that they obscure a clear view of the galaxy's stars, whose motion can be used to estimate the galaxy's mass.

To cut through the haze, Chris Carilli of the National Radio Astronomy Observatory in Socorro, New Mexico, and colleagues used radio telescopes to measure the motion of gas swirling around the centres of four distant quasars. The quasars are so far away that they appear as they were when the universe was less than 2 billion years old.

Small bulges

While the bulges of nearby galaxies all seem to be some 700 times as massive as their black holes, the four quasars broke this rule. Their bulges are only 30 times as heavy as their black holes, which weighed roughly a billion Suns.

"This suggests the black holes came first," Carilli said on Wednesday at a meeting of the American Astronomical Society in Long Beach, California.

The galaxies surrounding them must have had a significant growth spurt over the lifetime of the universe if they became galaxies similar to those seen in the present-day cosmos. But how this growth was accomplished is not clear, Carilli said.

Major puzzle

A fleet of new telescopes, including the Atacama Large Millimeter/Submillimeter Array, could give astronomers a more detailed view of the gas in quasars and allowing them to refine their estimates of the masses of early galaxies.

The origin of supermassive black holes is a major puzzle. Quasars have been spotted at times when the universe was less than a billion years old, which leaves a narrow window in which such black holes could form.

One idea suggests black holes grew from collapsed stars some 100 times the mass of the Sun. But the universe's first stars were not born until a few hundred million years after the big bang, which does not leave much time for such small black hole 'seeds' to grow.

An alternative possibility is that these black holes collapsed directly from massive gas clouds to form black holes with 1000 to 1 million times the mass of the Sun.

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Astronomers used the Very Large Array of radio telescopes to study the gas in this galaxy as it appeared just 870 million years after the big bang (Image: NRAO/AUI/NSF)